Understanding the anomalous temperature data from the Large Block Test at Yucca Mountain, Nevada

Understanding the anomalous temperature data from the

Large Block Test at Yucca Mountain, Nevada

S. Mukhopadhyay and Y. W. Tsang

E. O. Lawrence Berkeley National Laboratories, Berkeley, California, USA

Received 13 November 2001; revised 27 March 2002; accepted 15 April 2002; published 23 October 2002.

[1] The Large Block Test (LBT) at Yucca Mountain, Nevada, is unique because of itssize, which is large enough to include a number of large fractures, while still small enoughso that boundary conditions and heterogeneities can be adequately controlled orcharacterized. Extensive mapping of the test block has established the presence of manysmall and large fractures. Preheat air injection testing has also revealed that the block ishighly heterogeneous, with fracture permeability varying more than four orders ofmagnitude. We hypothesize that these large fractures and the resulting heterogeneity play asignificant role in the development of coupled thermal-hydrological (TH) processes in theLBT. A large volume of TH data, including temperature and saturation measurements,has been collected from the LBT. Some of these temperature data from the LBT,particularly those recorded by sensors TT1-14 and TT2-14, appear anomalous. We showthat these anomalous temperature data can be understood only if heterogeneity is invoked.As an example, we show that the heat pipe signature, an indication of the extent ofTH coupling, in TT2-14 is significantly longer in duration than that in TT1-14. We showthat such a difference can be explained only if one includes a high-permeability layer, theexistence of which has been indicated by air injection testing, through the location ofsensor TT2-14. Similarly, the oscillating temperature pattern in all sensors of boreholeTT2 during 4470–4500 hours of heating is shown to be the result of rainwater flowingdown a high-permeability fracture. The irregular temperature pattern in borehole TT1happens because of rainwater reaching the source of heat quickly through a highlypermeable, inclined fracture and vapor rising upwards through a less permeable verticalfracture. We also demonstrate that the anomalous temperature pattern recorded by TT1-14during 2500–3500 hours of heating can be explained in terms of continuous upwardmovement of vapor and the downward flow of condensate through the fractures. Weconclude that the temperature data from the LBT are the results of coupled TH processesoccurring in a heterogeneous environment. INDEX TERMS: 1829 Hydrology: Groundwater

hydrology; 1832 Hydrology: Groundwater transport; 1875 Hydrology: Unsaturated zone; KEYWORDS:

thermal, hydrological, Large Block Test, unsaturated flow, fractures, Yucca Mountain, Nevada

Citation: Mukhopadhyay, S., and Y. W. Tsang, Understanding the anomalous temperature data from the Large Block Test at Yucca

Mountain, Nevada, Water Resour. Res., 38(10), 1210, doi:10.1029/2001WR001059, 2002.

1. Introduction

[2] The Large Block Test (LBT) is one of the threethermal tests either completed or being carried out at YuccaMountain, Nevada. These thermal tests constitute an impor-tant part in the wider scientific site-characterization programat Yucca Mountain to determine its suitability as thepotential site for permanent geologic disposal of high-levelradioactive wastes. They have been designed for obtainingthermal-hydrological-mechanical-chemical (THMC)responses, which will help in developing a better under-standing of the THMC processes likely to exist in the rockmasses surrounding the potential repository [Tsang andBirkholzer, 1999; Birkholzer and Tsang, 2000; Mukhopad-hyay and Tsang, 2002].

[3] Two of those three tests (the Single Heater Test andthe Drift Scale Test) were underground in situ tests on thescales of meters to tens of meters. The LBT was aboveground and was unique in many ways. The test block, witha size of 3 � 3 � 4.5 m, was mechanically excavated fromthe surrounding Topopah Spring fractured nonlithophysaltuff in the Fran Ridge area of Yucca Mountain. The bottomof the block remained attached to the underlying rock, whilethe top and the four side faces were kept open to theatmosphere [Lin et al., 1994a, 1994b; Wilder et al., 1998].The size was large enough to include several large fractures,while still small enough such that boundary conditions androck heterogeneities could adequately be controlled orcharacterized. This intermediary scale of the LBT, betweena regular laboratory-scale test block and large-scale in situtests, allowed the study of coupled thermal-hydrological-mechanical (THM) processes under heating in a realistic,though somewhat controlled, environment.

Copyright 2002 by the American Geophysical Union.0043-1397/02/2001WR001059

28 - 1

WATER RESOURCES RESEARCH, VOL. 38, NO. 10, 1210, doi:10.1029/2001WR001059, 2002

[4] Heating in the LBTwas initiated on 28 February 1997and continued till 10 March 1998 (375 days of heating).Heating was then turned off and the block was allowed tocool. The cooling in the LBT was monitored till September1998. THM responses of the rock and pore water underheating, and subsequent cooling, were captured throughboth passive monitoring and active testing. A series ofvertical and horizontal boreholes were drilled into the blockand instruments were installed in them for collection ofTHM data [Wilder et al., 1998]. The automated passivemonitoring data acquisition included measurements of tem-perature, pressure, displacement, heating power, and volt-age. Periodically performed active testing included neutronlogging, acoustic emission and air permeability.[5] A more comprehensive description of the design,

construction, instrumentation and data collection systemsfrom the LBT is given by Lin et al. [1994a, 1994b], Lee[1995a, 1995b], Wilder et al. [1998] and Civilian Radio-active Waste Management System (CRWMS) [2000]. Pretestpredictive analysis of the thermal and hydrological responsefrom the LBT are given by Lee [1995a, 1995b] and Wilderet al. [1998], whereas limited comparative analysis ofthermal and hydrological data collected from LBT withmodel results are given by CRWMS [2000]. However, theseprevious modeling studies of the LBT have not focused onunderstanding a small portion of the temperature data,which appear anomalous at first glance. On closer exami-nation, these apparent irregularities can be attributed tospatial heterogeneiteis in rock properties affecting the evo-lution of the thermal-hydrological processes in the testblock. In this paper, we present a three-dimensional (3-D)numerical model to simulate the coupled transport of water,vapor, air, and heat for facilitating a better comparison ofsimulated and measured thermal and hydrological responsesfrom the LBT. In particular, we focus on developing a betterunderstanding of those seemingly anomalous and irregulartemperature data from the LBT.[6] Air permeability tests before initiation of heating

[Wang and Ahlers, 1997] in the LBT block have indicatedthat the fracture permeability in the block exhibits largespatial variability, with the measured permeability varyingfour orders of magnitude (10�15–10�11 m2) inside theblock. Mapping of all the fractures from the four sides

and the top surface of the block using a 1 ft � 1 ft gridsystem [Wagoner, 2000] yielded more than 2400 fractures,90% of which were less than 0.75 m long (43% beingshorter than 0.15 m). However, there were approximately 49fractures longer than 2 m, and of these, 11 were longer than4 m and at least one longer than 5 m. Most of these largefractures were found in a single near-vertical set [Wagoner,2000]. Figures 1a and 1b show some of these majorfractures in the block. The presence of such large-scalefractures, and the resulting heterogeneous permeabilitydistribution in the block, indicates that the flow of waterand heat-induced vapor can be very different in differentzones of the block, with vapor and water flowing moreeasily through the highly permeable fractures than else-where. It is our hypothesis that the seemingly anomaloustemperature data, the principal focus of this article, arise outof thermal-hydrological processes taking place in the het-erogeneous fracture system. We show, through simulationsperformed with our 3-D numerical model, that a heteroge-neous fracture system can indeed give rise to such irregularand seemingly anomalous temperature data.

2. LBT Configurations

[7] Figure 2 shows a schematic of the LBT block. Electricheaters were placed in five 2.44-m-long parallel boreholes ina horizontal plane 2.75 m below the top of the block (1.65 mabove the base of the block). The heater holes run approx-imately east (front face in Figure 2) to west, and each heateris supplied with a nominal electrical power of 450 W (with atotal nominal heating power of 2.25 kW). Spacing betweentwo adjacent heater holes was approximately 0.6 m. Theblock was covered with heavy insulation materials on allfour sides; however, the top surface was not insulated, andheat exchangers were installed to maintain the temperature atthe top surface at a constant 60�C. Figure 2 also shows thelocations of vertical and horizontal instrumentation bore-holes for monitoring temperature (labeled TT) and moisturecontent by neutron logs (labeled NEU). The TT and NEUboreholes are 1.5 inches (0.0375 m) in diameter and sealedby cement grout. Of particular interest for the discussion inthis article are the vertical temperature measurement bore-holes TT1 and TT2.

Figure 1. Fracture trace map on the surfaces of a physical model of the LBT: Views from the (left)southwest and (right) northeast [from Wilder et al., 1998].

28 - 2 MUKHOPADHYAY AND TSANG: ANOMALOUS TEMPERATURE DATA FROM LBT

[8] TT1 is located 1.22 m from the north face (right-handside in Figure 2) and 1.83 m from the east side (front face inFigure 2). There are 30 resistance temperature devices(RTD) in this borehole, TT1-1 to TT1-30. TT1-1 is at thebottom of the block, whereas TT1-30 is at the top of thehole. With the spacing between individual RTDs at 0.2 m,TT1-29 and TT1-30 are actually above the block. TT2, theother vertical borehole with RTDs, is similar to TT1 inevery respect except that it is located 1.82 m from the northside and 0.61 m from the east side of the block. Most of theanalysis in this article will revolve around the temperaturedata recorded by the two RTDs, TT1-14 and TT2-14. The

former RTD is located at 0.05 m above the plane of theheater holes, the latter is 0.05 m below it.

3. Temperature Data and Anomalies

[9] Figure 3 shows the temperature data recorded bysensors TT1-14 and TT2-14 through the entire duration of375 days (9,000 hours) of heating and 190 days (4,560hours) of cooling thereafter. Prior to commencement ofcooling at 9,000 hours, temperatures were brought downthrough a gradual reduction in power input beginning atabout 5,300 hours. This was done to prevent the temper-

Figure 2. Schematic representation of the test block in the LBT (not to scale).

MUKHOPADHYAY AND TSANG: ANOMALOUS TEMPERATURE DATA FROM LBT 28 - 3

ature in the block from exceeding the target maximumtemperature of 140�C. Input power was reduced at the rateof 4 W per heater per day for about a week and thereafter atthe rate of 1 W per heater per day for next three weeks.After this period of reduction, power input was broughtback up again to its original level. Apart from this, therewere several instances of power outages of short duration.These are reflected in sharp downward spikes in the temper-ature data. The most prominent one of those outages is theone that occurred at around 550 hours and lasted for about30 hours.[10] The sharp drop and subsequent oscillation of temper-

ature data in both TT1-14 and TT2-14 at around 4,470hours are not due to any power outage. We believe that theyare associated with a thunderstorm event that occurred on 2September 1997 (at 186 days of heating). That event musthave caused infiltration of rainwater down the fractures,even down to the plane of the heaters (as is shown by TT2-14) in the test block. The sharp drop in temperature to thenominal boiling point of 96�C, and the subsequent oscillat-ing patterns recorded by TT1-14, can be attributed torepeated cycles of drainage of cool, condensed water tothe heater horizon and subsequent vaporization of thatwater. TT1-14 also recorded a highly oscillating temper-ature pattern during 2,500–3,500 hours of heating. Whilethis cannot be correlated to a rain event, such a pattern maybe the result of a dynamic exchange process in a heteroge-neous environment (see last two paragraphs of section 5). Inthe ensuing discussion, we will show that, to simulate theseseemingly anomalous temperature signatures, we mustaccount for the heterogeneity of the fracture system.

4. Conceptual and Numerical Models

[11] We use a dual-permeability (DKM) conceptualiza-tion [Pruess, 1991] to represent the fractured tuff, with itslow-permeability rock matrix and the network of highlypermeable fractures embedded in it. In this DKM conceptu-alization, the rock matrix and the fractures are treated as twoseparate interacting continua. The numerical model for the

LBT, using the DKM conceptualization, is developed usingthe finite-integral simulator TOUGH2 [Pruess, 1987, 1991]for multidimensional coupled transport of water, vapor, air,and heat in porous and fractured media. TOUGH2 accountsfor the movement of liquid and gaseous phases, transport ofsensible and latent heat, and phase transitions betweenliquid and vapor. The physical processes of capillary suctionand adsorption in the liquid phase, binary diffusion in thegas phase, and vapor pressure lowering caused by capillaryand phase-adsorption effects are all included in the simu-lator. Thermal conductivity at a given saturation is obtainedby interpolating between ‘‘wet’’ and ‘‘dry’’ thermal con-ductivities, with a functional dependence on the square rootof saturation.[12] The grid for numerical simulation of the thermal and

hydrological processes in the LBT was developed by care-fully following the actual test geometry. The origin of thenumerical grid is located at the center of the central heaterborehole and at the east face of the test block (see Figure 2).First, a two-dimensional x-z grid was generated per thegeometry of the test for only the rock matrix. The grid wasrefined and radial in nature around the five heater holes, andthen gradually changed into a coarser and more rectangulargrid away from them. The bottom boundary of the numer-ical grid was located at 6.5 m below the heater plane,sufficiently far from it such that a constant boundarycondition could be applied there. The 3-D grid for the rockmatrix was generated through stacking six such x-z gridstogether in the y direction, with each x-z grid having athickness of 0.5 m. For the DKM conceptualization, iden-tical 3-D grids were generated for the fractures, and theentire interfacial area between the fractures and the matrixwas assumed to be available for fracture-matrix interflow.The complete 3-D DKM grid had 16,500 elements and52,074 connections between those elements. Figure 4ashows a schematic of a 2-D vertical section from the 3-Dnumerical grid, whereas a close-up view of the grid aroundthe central heater is shown in Figure 4b.[13] Key thermal and hydrological parameters used in the

numerical simulations of the LBT are tabulated in Table 1.Because the test block in the LBT has been excavated fromthe Fran Ridge area, which has a geology similar to themiddle nonlithophysal (Tptpmn) stratigraphic layer ofYucca Mountain, the property values are those of thatstratigraphic unit as derived from the Yucca Mountain SiteScale Model [Bodvarsson et al., 1997]. These rock proper-ties are similar to those used for modeling the in situ SingleHeater Test in the middle nonlithophysal unit of YuccaMountain (further details about the choice of rock propertyvalues are given by Tsang and Birkholzer [1999]). It willsuffice to mention here that isotropy is assumed for allproperties. Possible chemical and/or mechanical alterationsin rock properties owing to the effects of heating have notbeen included in the present model. Furthermore, rockproperties are assigned to all boreholes, thus making theimplicit assumption that wiring, grouting, and instrumenta-tion in the test block does not affect evolution of thermal-hydrological behavior in the test block. The block was opento the atmosphere at the top, with the top surface beingmaintained at a fixed temperature of 60�C. The four sides ofthe block, however, were insulated. In the numerical model,it is assumed that the four sides of the block are closed to

Figure 3. Measured temperatures from sensors TT1-14and TT2-14 of the LBT through the entire duration of thetest.


flow of heat by conduction (by applying zero thermalconductivity to the insulation materials). However, as theywere open to flow of moisture and vapor, a small amount ofheat may have been lost by convection. In the real test, it ispossible that a small amount of heat was also lost byconduction due to nonzero thermal conductivities of theinsulation materials. In the numerical model, constantpressure and temperature boundary conditions are appliedto all the boundaries of the block.

[14] Following the design criteria of the LBT, one elec-trical heater with a nominal power output of 450 W wasinstalled in each of the five heater holes. Most of theoccasional power outages were of small enough durationsuch that they did not have much impact on the testoutcome. A power outage beginning at around 550 hours(lasting approximately 30 hours), whose impact was clearlyrecorded by both TT1-14 and TT2-14, was included explic-

itly in the numerical simulations. We therefore separated thetotal heating period into three stages. The first stage lastedthrough 550 hours of heating and the third stage lasted from580 hours of heating through to switching-off of power. Thefirst and third stages of heating were simulated using theaverage of actual power input, making it unnecessary toexplicitly account for the individual minor power outages orthe intentional reduction in power beginning at about 5,300hours.

5. Model Results and Measured Data

[15] The first set of simulation results assumes that thefractures form an isotropic and homogeneous continuum. InFigures 5a and 5b, we show contours of temperature at 50days and 104 days of heating in the vertical plane ofborehole TT2. This plane is 0.5 m thick and centered 0.75m from the east side of the block, while borehole TT2 isactually located 0.61 m from that same side. During theearly phase of heating, the hot zones around the individualheaters are distinct (Figure 5a). Away from the heaters, bothabove and below, there is a gradual decrease in temper-atures. The bottom boundary of the block, being fartheraway from the heater plane than the top, is considerablycooler than the top, which also has an imposed constanttemperature boundary. With progress of heating, hot zonesaround individual heater extend farther and are not individ-ually identifiable after a certain time (Figure 5b).[16] Heating in the LBT gives rise to redistribution of

moisture in the test block. As the formation temperaturesapproach 96�C (the nominal boiling point of water at theprevailing pressure) around the heater, matrix pore waterboils and vaporizes. Most of the vapor generated moves intothe fractures, where it becomes highly mobile and is drivenwith the gas pressure gradient away from the heat source.When the vapor encounters cooler rock, it condenses, andthe local fracture saturation builds up. Part of the condensatemay then imbibe into the matrix, where it is subject to avery strong capillary gradient toward the drier regionaround the heat source, giving rise to a reflux of liquidback to the heaters. If matrix imbibition is relatively slow,

a

b

Figure 4. Sample numerical grid used in simulating thethermal-hydrological processes in the LBT. (a) A verticalsection of the three-dimensional grid and (b) a close-upview of the same around the central heater.

Table 1. Hydrological and Thermal Input Values a

Parameter Value

Matrix porosity 0.11Matrix permeability 4.0 � 10�18 m2

Matrix van Genuchten parameter a 6.4 � 10�7 Pa�1

Matrix van Genuchten parameter b = 1/(1-m). 1.47Matrix residual liquid saturation 0.18Matrix grain density 2540.0 kg/m3

Initial matrix liquid saturation 0.92Fracture porosity 0.000243Fracture continuum permeability 5.85 � 10�14 m2

Fracture van Genuchten a 1.0 � 10�3 Pa�1

Fracture van Genuchten b 1.47Fracture residual liquid saturation 0.01Rock mass thermal conductivity Cdry = 1.67 W/(m-K)C(Sl) = Cdry + (Cwet � Cdry)

ffiffiffiffi

Sip

Cwet = 2.0 W/(m-K)Rock mass heat capacity 953.0 J/(kg-K)Vapor diffusion coefficient Do

va 2.14 � 10�5 m2/sTemperature dependence q 2.334Fracture spacing 0.53 m

aExcerpted from Tsang and Birkholzer [1999].


the condensate may build up in the fractures and eventuallybecome mobile. Some fraction of the condensate in thefractures may flow back toward the drier region. However,because capillary forces are relatively weak in the fractures,a substantial amount of liquid may drain by gravity (downtoward the heater from condensation zone above the heaterhorizon, and down away from the heater from condensationzone below the heater horizon). Such a countercurent flowof vapor and condensate results in what is known as the‘‘heat pipe’’ signature, a period of constant temperature at agiven location [White et al., 1971]. Gravity drainage, andthe duration of the heat pipe, depends on the strength ofevaporation-condensation and fracture-matrix interflowbehavior. The stronger the vapor flux away from the heaterand the condensate reflux toward the heater, the moreprominent the heat pipe signature.[17] Such heat-driven moisture redistribution in the LBT

is demonstrated in Figures 6 and 7, where the contours ofliquid saturation are shown for the fractures and the matrix,respectively. The contours of fracture liquid saturations inFigure 6a exhibit a dry-out zone above and below the heaterplane. However, unlike the contours of temperature, thefracture liquid saturations are rather asymmetric, withthicker condensate zones below the heater plane than above.As heating continues, the dry-out zone expands (Figure 6b).In addition, the condensate zone below the heater planebecomes larger, while the condensate zone above has starteddisappearing. These results seem to indicate gravity-driven

flux in the fractures, giving rise to the significantly wetterzone below the heater horizon than above. The presence ofthe wetter zone normally manifests itself in the form of aheat pipe signature in the temperature profile, as liquid boilsat constant temperature. Thus sensors in the condensatezone below the heater plane exhibit a heat pipe signature oflonger duration than by those above.[18] Figure 7a shows a dry-out zone in the matrix around

the heater plane. The wetter zone is rather small comparedto that in the fractures. This small condensate zone is rathersymmetrically distributed above and below the heater plane,particularly at 50 days of heating. At 104 days of heating(Figure 7b), the dry-out zone has expanded, and the con-densate zone has begun to exhibit a slight asymmetry, with amarginally thicker condensate zone below the heater planethan above. Comparison with the very asymmetric moisturedistribution in the fractures (as shown in Figures 6a and 6b)indicates that the gravity drainage of condensate in thefractures is faster than the imbibition of condensate fromthe fractures into the matrix. This can be directly attributedto the considerably higher (four orders of magnitude)permeability of the fractures compared to that of the rockmatrix.[19] Figure 8a compares measured and simulated temper-

atures in the location of sensor TT1-14 through the entireheating and cooling periods of the LBT. Simulated temper-atures match the measured ones reasonably (except for theanomalies during 2,500–3,500 hours, which will be dis-

Figure 5. Contours of temperature in the vertical plane containing borehole TT2 at (a) 1200 hours (50days) and (b) 2500 hours (104.17 days) of heating.


Figure 7. Contours of matrix liquid saturation in the vertical plane containing borehole TT2 at (a) 1200hours (50 days) and (b) 2500 hours (104.17 days) of heating.

Figure 6. Contours of fracture liquid saturation in the vertical plane containing borehole TT2 at (a)1200 hours (50 days) and (b) 2500 (104.17 days) hours of heating.


cussed later). The model captures the early conduction-driven linear rise in temperature, and it also produces a heatpipe signature of short duration, consistent with the data.This implies that the model is able to capture the complexthermal and hydrological processes occurring in the LBT.Measured and simulated temperatures between 5,300 hoursand the start of cooling do not match as closely as else-where, because the model did not explicitly account for thestepwise reduction in power, but used an average powerinstead. The model also reproduces the temperatures duringthe cooling cycle reasonably well. The slight increase inmeasured temperatures toward the end of the cooling periodmay have resulted from seasonal changes in ambienttemperatures. But these variations in ambient conditionsare not considered significant and have not been included inthe model.[20] Figure 8b shows a similar comparison of measured

and modeled temperatures in the location of sensor TT2-14.While the model adequately reproduces the measured tem-peratures, there is a considerable discrepancy between themeasured and simulated duration of heat pipe signature atthis location. Also, the duration (about 200 hours) of theheat pipe signature at TT1-14 (Figure 8a) is significantlysmaller than that at TT2-14 (about 2000 hours in duration).This can partly be due to their locations. TT1-14 beinglocated 0.05 m above the heater plane, while TT2-14 islocated 0.05 m below the heater plane. It has been shown(refer to Figures 6a and 6b) that owing to faster gravitydrainage, there is a thicker buildup of condensate zonebelow the heater plane than above. The thicker condensatezone translates to a longer heat pipe signature below theheater plane than above. Indeed, the modeled heat pipeduration at TT2-14 (Figure 8b) is slightly longer than themodeled heat pipe duration at TT1-14 (Figure 8a). How-ever, the difference is much smaller than that exhibited bythe measured heat pipe duration from those locations.[21] The very long heat pipe signature at TT2-14 may

have resulted from local heterogeneity. Air injection tests[Wang and Ahlers, 1997] conducted in a vertical boreholenear the center of the block, before the block was excavated,indicated a distinct, very high-permeability zone immedi-ately below the heater plane (Figure 9). We repeated the

Figure 9. Measured pretest air permeability variations along a vertical borehole near the center of theblock [from Wang and Ahlers, 1997].

Figure 8. Comparison of measured and simulated tem-peratures through the entire cycle of heating and cooling inthe LBT at the location of (a) TT1-14 and (b) TT2-14.


simulations of Figure 8b, except that a horizontal layer (ofthickness 0.1 m) was now included through the location ofsensor TT2-14, with fracture permeability of this layerbeing four orders of magnitude higher than those of thebase case property set. This is consistent with the observedheterogeneity in permeability values (as shown in Figure 9).The simulation that incorporates the high-permeability fea-ture gives rise to a heat pipe signature of considerablylonger duration than that with base-case permeability and ismore consistent with the measured heat pipe signature atTT2-14 (Figure 10). The larger permeability imposed at thelocation of TT2-14 brings in more vapor for condensation,and its horizontal orientation prevents condensate fromdraining away. We also evaluated the situation of a verticalhigh permeability layer instead of a horizontal one. This didnot produce a comparable heat pipe signature at TT2-14.This confirms that the long heat pipe signature at TT2-14may be the result not only of gravity-drainage (because ofits location below the heater plane) but also of the perme-ability heterogeneity (and its orientation) at that location.[22] The above result leads us to inquire whether hetero-

geneity may also be the cause of the seemingly anomaloustemperature data in boreholes TT1 and TT2 following therain event, that at approximately 4,470 hours of heating.The analysis of temperature data from TT2 (Figures 11a and11b) is presented first. Figure 11a shows the temperaturedata recorded by sensors TT2-28 to TT2-14 during the rainevent, while Figure 11b shows the same for sensors TT2-14to TT2-1. sensors TT2-28 to TT2-15 are located above theheater plane and sensors TT2-13 to TT2-1 are below it.Sensor TT2-14 is actually located 0.05 below the heaterplane, and temperature data recorded by this sensor havebeen included in both Figures 11a and 11b. Sensor TT2-28,recording a near-constant temperature of 60�C before therain event, registered a sharp fall in temperature beforeslowly recovering over a period of about 35–40 hours.While the same pattern is recorded by sensor TT2-27, thedrop in temperature is much less prominent than at TT2-28.Sensors TT2-26 through TT2-20 record virtually no changein temperature during that period of time. A possible

explanation for this is that the temperature (below boiling)of the rain water reaching these locations must have beensuch that a thermal equilibrium has been maintained (it isalso possible that these sensors were not in the path of therain water). However, sensors TT2-17 through TT2-14,which were originally well above the boiling point of water,recorded sharp decreases in temperatures down to thenominal boiling point. Sensors TT2-13 and TT2-12 (Figure11b) registered drops in temperatures during the rain event.However, none of the other sensors below the heater planerecorded any drop, indicating that rain water possibly didnot penetrate past the location of TT2-12. The temperaturesignatures from all the sensors in borehole clearly demon-strate water trickling down some highly permeable path.[23] Having argued the plausible process that can result in

the trends of temperatures recorded in borehole TT2, wenow present the numerical simulations. A major uncertaintyin modeling the thermal and hydrological events associatedwith the rain event is the lack of information about theactual duration of the event, the total amount of rainfallFigure 10. Impact of local fracture permeability hetero-

geneity on the heat pipe signature at TT2-14.

Figure 11. Measured temperature data from all sensors inborehole TT2 immediately before and after the rain event(a) for sensors above the heater plane and (b) for thosebelow the heater plane.


during that period, and the amount of water that actuallyinfiltrated the test block. In the absence of concrete data, weperformed simulations based on likely scenarios.[24] To invoke heterogeneity, we included a thin discrete

vertical fracture in an otherwise dual-permeability contin-uum model. This vertical fracture is 0.001 m thick and islocated 1.8 m from the north side and 0.75 m from the eastside of the block (TT2 is located 1.82 m from the north sideand 0.61 m from the east side of the block). This fracturehad permeability two orders of magnitude higher than thepermeabilities of the fractures elsewhere in the test block.An inch of rainwater was then allowed to fall on the blockfor a period of 24 hours. A close match between simulatedand measured temperatures can be obtained with the inclu-sion of a permeable fracture in the DKM model (Figure 12).Sensitivity studies performed with various rainfall amountand fracture permeabilities revealed that an unrealisticamount of water would be required to obtain a similarmatch if we used only the DKM (no high-permeabilityfeature). On the other hand, increasing the permeability ofthe heterogeneous feature by a couple of orders of magni-tude would have necessitated a smaller infiltration rate. Thisonly establishes that, to capture the temperature signature inborehole TT2, heterogeneity needs to be invoked. Wehypothesized at the beginning of this paper that the seem-ingly anomalous temperature data arise from dynamicinteractions between the physical processes involved inheating fractured rock and the heterogeneities in the rock.It is clear, even with the simple heterogeneity considered inthis numerical model, that such dynamic exchange pro-cesses have been captured, certainly qualitatively if notquantitatively.[25] Measured temperature data from all sensors in bore-

hole TT1 during the rain event are presented in Figure 13.The rain-related temperature excursions in borehole TT1 arefundamentally different from those in borehole TT2 (seeFigure 11). For example, compare the temperature signa-tures at TT1-28 (Figure 13a) and TT2-28 (Figure 11a), bothlocated near the top of the block. Sensor TT1-28, at 60�C

before the advent of rain, continued to remain at 60�C evenafter it started raining, before sharply rising to the boilingpoint of water. It then stayed at the boiling point for sometime, before declining gradually back to its original temper-ature. This is in contrast to TT2-28, where the temperaturedecreased sharply after coming in contact with cooler rainwater at the beginning of the rain event. Other sensors inborehole TT1 that were registering temperatures belowboiling increased to the nominal boiling point of water,and those above boiling went down to the boiling point. Itappears that in borehole TT1 liquid water managed tobypass some of the sensors above the heater plane. Onceat the heater plane, the water rapidly vaporized, and thevapor moved upwards through some permeable path closeto the location of borehole TT1. The cooling effect of thisupward-moving vapor may explain the almost immediatedrop in temperature at sensor TT1-14 (from approximately140oC to the boiling point of water), while temperature atthe top of the block slowly increased from 60�C to theFigure 12. Comparison of measured and simulated (with a

discrete fracture embedded in a DKM model) temperaturesat selected sensor locations of borehole TT2 immediatelybefore and after the rain event.

Figure 13. Measured temperature data from all sensors inborehole TT1 immediately before and after the rain eventfor (a) sensors above the heater plane and (b) those belowthe heater plane.


boiling point of water over a period of approximately 30hours.[26] To reproduce the temperature data in borehole TT1,

we introduced an inclined (with an angle of inclination of75� with the horizontal axis) fracture in our DKM modeldomain with permeability four orders of magnitude higherthan that shown in Table 1. This provides the fast pathnecessary to bring the rainwater quickly down to the heaterhorizon. A vertical feature, with permeability two orders ofmagnitude higher than the base-case fracture permeability,was also introduced near the location of borehole TT1. Thisvertical feature was similar to the one introduced for TT2.The vertical fracture intersected the inclined fracture at alocation close to that of sensor TT1-14 and provided a fastpath for vapor to reach the sensors above the heater plane inborehole TT1. In Figure 14, comparisons of measured andsimulated temperatures are shown for selected sensors inborehole TT1. The model qualitatively reproduces almost allthe characteristics of the temperature data in borehole TT1. Itis also to be noted that simulations performed with ahorizontal high permeability layer (such as the one discussedearlier), in addition to the two fractures discussed here,resulted in similar temperature profiles in borehole TT1.[27] The analysis presented so far clearly indicates that

local heterogeneity, due to the presence of discrete fractures,played a significant role in influencing the temperature datacollected from the LBT. Given the heterogeneous nature ofthe block, with many small- and large-scale fractures, this isnot surprising. While it is not feasible to develop a modelincorporating every fracture in the test block, owing toobvious constraints on availability of fracture data and oncomputational efforts, we have demonstrated that most ofthe major features of the anomalous temperature data fromthe LBT can be understood and reproduced adequatelythrough selective inclusion of local heterogeneities.[28] However, we made no attempt to model the irregular

temperature data during 2500–3500 hours of heating (par-ticularly the oscillating data recorded by TT1-14, seeFigure 3). These patterns recorded by TT1-14, a very sharp

drop in temperature to the boiling point and subsequentincrease through multitudes of small-scale oscillations, areindicative of those phenomena similar to those that gave riseto the temperature signatures in TT2 during the rain event at4,470 hours of heating. However, to the best of our knowl-edge, no rain event occurred during 2,500–3,500 hours ofheating. Had there been a rain event at that time, its impactwould presumably have been recorded by some of thesensors in TT2.[29] We believe that these temperature signatures

recorded by TT1-14 are the results of a dynamic exchangeprocess, specifically a rapid drainage of condensate(through highly permeable fractures) down to the sourceof heat, followed by rapid evaporation of that water awayfrom the heater plane. It is plausible that the rate of heatingand the thermal and local hydrologic properties of the rockwere such that a just-sufficient amount of condensaterefluxed back to the heat source, and that the entire amountwas boiled away. The amount of refluxing condensate wasnot sufficient to overcome the thermal barrier around theheaters and drain below them. This dynamic exchangeprocess might have been sustained over a long period oftime, resulting in the oscillating patterns until all the waterwas displaced. This hypothesis of dynamic exchange proc-ess is supported by the fact that TT2-14, which is locatedbelow the heater plane, recorded none of these oscillatingpatterns of TT1-14 during that time. This phenomenon alsoseems similar to the geysering process simulated by Inge-britsen and Rojstaczer [1993, 1996].

6. Summary and Conclusions

[30] The LBT was conducted as part of the thermal testprogram at the Yucca Mountain to acquire a more in-depthunderstanding of the THMC responses likely to exist in thefractured welded tuff rock masses surrounding the potentialrepository. To analyze the temperature data from the test, a3D numerical model, based on the TOUGH2 finite-integralsimulator and dual-permeability conceptualization of rockmatrix and fracture interactions, was developed. The modelhas captured most of the thermally driven hydrologicalprocesses in an unsaturated fractured rock. The temperaturedata from the LBT present a unique opportunity to furtherour understanding of the impact of discrete fracture hetero-geneity on thermally driven liquid and gas flow.[31] Although the model treats the fractures as a homo-

geneous continuum, and can in general reproduce thetemperature measurements (including heat pipe signatures,indicating water and vapor phase counter flow), there is asmall portion of temperature data that shows irregularity andappears anomalous. Upon further study, these apparentanomalies are attributable to the impact of heterogeneityarising out of the presence of discrete fractures. Detailedmapping of the LBT block confirmed presence of severalsuch discrete fractures; their effects on flow were capturedin preheat air permeability measurements that indicatedlarge spatial variability, with permeability values rangingover four orders of magnitude. By incorporating some ofthese discrete fractures into the dual-continuum approach,the apparent anomalies in the temperature data were repro-duced in the modeled results.[32] Temperature history from sensor TT2-14, located at

0.05 m below the heater plane, registered a heat pipe

Figure 14. Comparison of measured and simulated (withdiscrete fractures embedded in a DKM model) temperaturesat selected sensor locations of borehole TT1 immediatelybefore and after the rain event.


signature significantly longer in duration than that recordedby sensor TT1-14, located 0.05 m above the heater plane.Predictions from the model treating the fractures as ahomogeneous continuum showed a heat pipe signature inTT2-14 only slightly longer (due to gravity drainage) thanthat in TT1-14. Based on the fracture maps and the airpermeability data, we established the presence of a layer ofhigh-permeability passing through the location of sensorTT2-14. This local heterogeneity gave rise to a heat pipesignature of duration similar to the measured duration of theheat pipe at TT2-14.[33] The seemingly anomalous temperature data around

4,470 hours of heating can be correlated to a rain event thatoccurred during that time. Rainwater trickled down somepermeable fracture close to borehole TT2, reducing thetemperature. We reproduced these data by introducing ahigh-permeability fracture in the dual permeability model.Because the local heterogeneity in the vicinity of TT1differs from that of TT2, the temperature histories of theclosely spaced sensors in TT1 told a different story. Inborehole TT1, the rain bypassed sensors near the top, wasable to reach the heated plane rapidly, and then evaporated.The vapor so generated moved upward through the verticalborehole, producing complicated temperature signatures fordifferent sensors. By introducing a high-permeabilityinclined fracture and another less permeable vertical fracturein the dual- permeability model, we reproduced thesetemperature signatures. We hypothesize that a similardynamic exchange between condensate draining downtoward the source of heat and vapor moving away fromthe heat source could bring about the oscillating patterns oftemperature recorded by sensor TT1-14 during 2,500–3,500 hours of heating.

[34] Acknowledgments. We thank the anonymous reviewers of theWRR for their careful and critical review of the manuscript. We thankCharles Haukwa, Eric Sonnenthal and Dan Hawkes of the Ernest OrlandoLawrence Berkeley National Laboratories for their constructive review ofthe manuscript. This work was supported by the Director, Office of CivilianRadioactive Waste Management, U.S. Department of Energy, throughMemorandum Purchase Order EA9013MC5X between Bechtel SAICCompany, LLC and LBNL. The support is provided to LBNL throughthe U.S. Department of Energy contract DE-AC03-76SF00098.

ReferencesBirkholzer, J. T., and Y. W. Tsang, Modeling the thermal-hydrologic pro-cesses in a large-scale underground heater test in partially saturated frac-tured tuff, Water Resour. Res., 36(6), 1431–1447, 2000.

Bodvarsson, G. S., T. M. Bandurraga, and Y. S. Wu (Eds.), The site-scale

unsaturated zone model of Yucca Mountain, Nevada for the viabilityassessment, Yucca Mountain Site Characterization Project Rep. LBL-40,376,UC-814, Lawrence Berkeley Natl. Lab., Berkeley, Calif., 1997.

Civilian Radioactive Waste Management System (CRWMS), Thermal teststhermal-hydrological analyses/model report, ANL-NBS-TH-000001 REV00, CRWMS Management and Operating Contractor (M&O), Las Vegas,Nev., 2000.

Ingebritsen, S. E., and S. A. Rojstaczer, Controls on geyser periodicity,Science, 262, 889–892, 1993.

Ingebritsen, S. E., and S. A. Rojstaczer, Geyser periodicity and the responseof geysers to deformation, J. Geophys. Res., 101, 21,891–21,905, 1996.

Lee, K., Progress report on pre-test calculations for the Large Block Test,Tech. Rep.UCRL-ID-118699, Lawrence Livermore Natl. Lab., Liver-more, Calif., 1995a.

Lee, K., Second progress report on pre-test calculations for the Large BlockTest, Tech. Rep.UCRL-ID-12230, Lawrence Livermore Natl. Lab., Liver-more, Calif., 1995b.

Lin, W., D. G. Wilder, J. A. Blink, S. C. Blair, T. A. Buscheck, D. A.Chesnut, W. E. Glassley, K. Lee, and J. J. Roberts, The testing of ther-mal-mechanical-hydrological-chemical processes using a large block,Tech. Rep. UCRL-JC-114776, Lawrence Livermore Natl. Lab., Liver-more, Calif., 1994a.

Lin, W., D. G. Wilder, J. A. Blink, S. C. Blair, T. A. Buscheck, R. S. Glass,W. E. Glassley, K. Lee, R. D. McCright, M. W. Owens, and J. J. Roberts,A Large Block Heater Test for high level nuclear waste management,Tech. Rep. UCRL-JC-116431, Lawrence Livermore Natl. Lab., Liver-more, Calif., 1994b.

Mukhopadhyay, S., and Y. W. Tsang, Uncertainties in coupled thermal-hydrological processes associated with the Drift Scale Test at YuccaMountain, Nevada, J. Contam. Hydrol., in press, 2002.

Pruess, K., TOUGH2 user’s guide, Tech. Rep. LBL-20700, Lawrence Ber-keley Natl. Lab., Berkeley, Calif., 1987.

Pruess, K., TOUGH2—A general purpose numerical simulator for multi-phase fluid and heat flow, Tech. Rep. LBL-29400, UC-251, LawrenceBerkeley Natl. Lab., Berkeley, Calif., 1991.

Tsang, Y. W., and J. T. Birkholzer, Predictions and observations of thethermal-hydrological conditions in the Single Heater Test, J. Contam.Hydrol., 38, 385–425, 1999.

Wagoner, J., Surface fracture data report for the Large Block Test, FranRidge, Yucca Mountain, Nevada, Tech. Rep. UCRL-ID-140784, Lawr-ence Livermore Natl. Lab., Livermore, Calif., 2000.

Wang, J. S. Y., and C. F. Ahlers, Pre-test analyses of the Large Block Test—Air-flow and gas tracer transport for thermal-hydrological evolution,Yucca Mountain Site Characterization Project Rep. SP9012M5,CRWMS Management and Operating Contractor (M&O), Las Vegas,Nev., 1997.

White, D. E., L. J. P. Muffler, and A. H. Truesdell, Vapor-dominated hy-drolthermal systems compared with hot-water systems, Econ. Geol., 66,75–97, 1971.

Wilder, D. G., W. Lin, S. C. Blair, T. A. Buscheck, R. C. Carlson, K. Lee,and A. Meike, Large Block Test status report, Tech. Rep. UCRL-ID-128776, Lawrence Livermore Natl. Lab., Livermore, Calif., 1998.

��S. Mukhopadhyay and Y. W. Tsang, Lawrence Berkeley National

Laboratory, MS 90-1116, 1 Cyclotron Road, Berkeley, CA 94720, USA.([email protected]; [email protected])


Understanding the anomalous temperature data from the Large Block Test at Yucca Mountain, Nevada

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